Optical interferometers are largely classified into those based on an amplitude-splitting method and those based on a phase-splitting method. Excluding specific cases, an optical interferometer (such as that based on a laser) generally employs an amplitude-splitting method. This is because a phase distortion caused by an optical system can be compensated and thereby it becomes relatively easy to accurately detect a minute phase distribution which is an object to be observed. On the other hand, an electron interferometer generally employs the phase-splitting method except in the cases described in Non-patent document 1 (Q. Ru et al.: Ultramicroscopy 53, 1 (1994)). This is because there is no effective amplitude-splitting type beam splitter for an electron beam.
As an electron interferometer, only a phase-splitting type interferometer having one electron biprism has been used. However, this type of interferometer cannot control each of a fringe spacing s and an interference width W independently because of its operational principles. For instance, when a specimen is large and a wide interference width is necessary, it is necessary to analyze an interference image (hologram) formed with small fringe spacing and many interference fringes, in other words, an image recorded with a high carrier-spatial frequency. On the contrary, even if a specimen is small and an interference image obtained in a narrow area with a high carrier-spatial frequency is required for analysis, when a necessary high carrier-spatial frequency is produced, the interference width becomes larger with the spatial coherence deteriorated. As a result, a (low-quality) interference image having the low-contrast interference fringes needs to be analyzed.
To solve the problems as described above, some electron optical systems have been developed or re-configured based on various researches and experiments. Due to some characteristics (for example, an optical system with only several convex lenses, and an optical system without a half mirror) of electron optical systems, however, there are limitations such as a limitation where the magnification finally obtained is low.
In addition, when a hologram is reconstructed and a phase image is extracted, the Fresnel diffraction waves caused by a breakage at the wavefront generate fringes with strong contrast (Fresnel fringes), which generates artifact in the result of the measurement and impedes high precision phase measurement. When an object to be observed is a weak phase object, some methods have been proposed such as a method in which the effects of phase distribution due to Fresnel fringes are corrected later (Refer to, for instance, Non-patent document 2: K. Harada et al.: J. Electron Microsc. 52, 369 (2003)), a method in which the effects are eliminated in the Fourier space upon regeneration, a method in which the effects are subtracted as strength from two holograms (Refer to, for instance, Non-patent document 3: K. harada and R. Shimize: J Electron Microsc. 40, 92, (1991)), and a method in which phase components of Fresnel fringes are extracted and subtracted, (Patent document 1: International publication No. 01/075394, pamphlet). In all of these methods, however, the effects of Fresnel fringes are removed after data is recorded. These methods are not sufficient. As a result, a practical method is to neglect the end of interference range where Fresnel fringes have strong contrast, even if an interference range is sacrificed. On the other hand, ideas of suppressing formation of Fresnel fringes are proposed, for instance, in Patent document 1 (International publication No. 01/075394, pamphlet). According to the document, to eliminate Fresnel fringes generation from a wavefront-splitting boundary, a beam stopper plate is placed on a plane equivalent to the observation plane so that the wavefront-splitting boundary of a wavefront-splitting device is placed in the shadow of the observation plane. Thereby, it is possible to have a case where Fresnel diffraction does not occur. This method, however, would not provide any improvement of the primary problem that each of fringe spacing s and interference width W cannot be controlled independently.
Patent Document 1: International publication 01/075394
Non-patent Document 1: Q. Ru et al.: Ultramicroscopy 53, 1 (1994)
Non-patent Document 2: K. Harada et al.: J. Electron Microsc. 52, 369 (2003)
Non-patent Document 3: K. Harada and R. Shimizu: J. Electron Microsc. 40, 92 (1991)